Field of the Invention
The invention concerns: the correction of measurement data acquired by magnetic resonance technology, the measurement data having been acquired by operation of a pulse sequence in which gradients are activated simultaneously during the radiation of at least one excitation pulse; and therefore also the correction of artifacts in magnetic resonance images that have been reconstructed from measurement data that have been acquired by an MR pulse sequence in which simultaneous gradients are activated during the radiation of at least one excitation pulse.
Description of the Prior Art
Magnetic resonance (MR) tomography is a known modality with which images of the inside of an examination subject can be generated. Expressed in a simplified manner, the examination subject is positioned in a magnetic resonance apparatus in a strong, static, homogeneous basic magnetic field (also called a B0 field) with a field strength of 0.2 to 7 Tesla or more, such that nuclear spins in the subject orient along the basic magnetic field. To trigger nuclear magnetic resonance, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject, the triggered nuclear magnetic resonances are measured as data are known as k-space data, and on this basis MR images are reconstructed or spectroscopy data are determined. For spatial coding of the measurement data, rapidly switched gradient fields (also simply called gradients for short) are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a memory organized as a k-space matrix. For example, an associated MR image can be reconstructed, by a multidimensional Fourier transformation, for example, from the k-space matrix populated with values.
A known pulse sequence to excite and acquire the nuclear magnetic resonances is the gradient echo sequence, in particular to acquire three-dimensional (3D) data sets. However, such gradient echo-based MR examinations are usually very loud, and therefore are uncomfortable for a patient to be examined. The primary reason for the high noise development is gradient configurations that change rapidly over time and the high slew rates (chronological change of the gradient amplitudes dG/dt associated therewith). Moreover, in the protocol of the sequence, parameters are often necessary that require particularly fast switching of the gradients, for example short echo times or gradient spoiling.
In spite of this, non-selective 3D gradient echo measurements are often used in clinical imaging, but primarily for what are known as preparation measurements. The most important example of such a preparation measurement is the coil sensitivity measurement that is implemented on this patient at least once before the beginning of the actual patient examination if an acquisition coil is used with a sensitivity profile that differs depending on use. During the coil sensitivity measurement, two 3D measurement data sets are acquired, wherein one measurement data set is acquired with what is known as the “body coil” integrated into the magnetic resonance system and the other measurement data set is acquired with the local coil to be used. The sensitivity profile of the local coil, and therefore the intensity distribution of the local coil image, can be calculated on the basis of these two measurement data sets, which includes a division of the two images reconstructed from the respective measurement data sets, and inhomogeneities that arise due to the relative distances from the measurement subject to be examined from the coil element or coil elements of the local coil.
In order to lower the noise volume of such measurements, the maximum gradient performance provided by the sequence can be decreased until the measurement is markedly quieter. However, the minimum echo time thereby increases, the maximum bandwidth possibly decreases, and the repetition time TR of the sequence and the measurement time increase, such that the result of the measurement is not optimal under the circumstances.
In order to reduce the noise development of a gradient echo sequence, for the phase coding and/or pre-dephasing, gradients to be activated are already ramped up before the excitation due to the excitation pulse occurs. More time therefore remains for additional gradients to be activated in the echo time, so these gradients can be activated with reduced slew rate and/or amplitude. However, an unwanted slice selection also occurs by this measure.
In the known PETRA sequence or the known z-TE sequence, the gradients are likewise already activated during the excitation. The spectral profile of the excitation pulse corresponds approximately to a sine function. In the case of insufficient pulse bandwidth or gradients that are too strong, it can also occur that the outer image regions are no longer sufficiently excited by the slice selection that occurs.
In the reconstructed MR image, this incorrect excitation is expressed by smearing artifacts at the image edge, which are more strongly expressed the more strongly that gradients are switched during the excitation.
An insufficient excitation thus leads to artifact-plagued MR images. For the PETRA sequence, from the article by Grodzki et al., “Correcting slice selectivity in hard pulse sequences”, Journal of Magnetic Resonance 214, P. 61-67 (2012), or also the US Patent Application US 20130101198 A1, a correction method is known with which such artifacts can be calculated and remedied utilizing the radial symmetry inherent to the PETRA sequence by a matrix inversion. For Cartesian scanning of k-space as is the rule for gradient echo sequences, three-dimensional gradient echo sequences in which k-space is scanned in parallel lines in the readout direction, such a correction is not possible due to the different symmetry.